Method of making bipolar transistor

ABSTRACT

A method of making a bipolar transistor includes patterning a first photoresist over a collector region of the bipolar transistor, the first photoresist defining a first opening. The method further includes performing a first implantation process through the first opening. The method further includes patterning a second photoresist over the collector region, the second photoresist defining a second opening different from the first opening. The method further includes performing a second implantation process through the second opening, wherein a dopant concentration resulting from the second implantation process is different from a dopant concentration resulting from the first implantation process.

PRIORITY CLAIM

The present application is a divisional of U.S. application Ser. No.14/056,393, filed Oct. 17, 2013, which is incorporated here by referencein its entirety.

BACKGROUND

Technological advances in semiconductor integrated circuit (IC)materials, design, processing, and manufacturing have enabledever-shrinking IC devices, where each generation has smaller and morecomplex circuits than the previous generation.

Bipolar transistors are used to selectively connect electricallyseparate devices in an integrated circuit. Performance of a bipolartransistor is measured using a turn-off time, a time period between thebipolar transistor receiving a turn off signal and the bipolartransistor becoming non-conductive. Performance of a bipolar transistoris also measured by a voltage drop across the bipolar transistor, whichis determined, in some instance, by an on-state resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a cross-sectional view of a bipolar transistor having a splitcollector region in accordance with one or more embodiments;

FIG. 2 is a cross-sectional view of a split collector region for abipolar transistor in accordance with one or more embodiments;

FIG. 3A is a top view of a bipolar transistor having a split collectorregion in accordance with one or more embodiments;

FIG. 3B is a top view of a split collector region for a bipolartransistor in accordance with one or more embodiments;

FIG. 4 is flow chart of a method of making a split collector region inaccordance with one or more embodiments; and

FIGS. 5A and 5B are cross-sectional views of a split collector regionduring various points of manufacturing in accordance with one or moreembodiments.

Various embodiments of the present invention will be explained in detailwith reference to the accompanying drawings.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are examples and are not intended to belimiting.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper”, “over” and the like, may be used herein for ease of descriptionto describe one element or feature's relationship to another element(s)or feature(s) as illustrated in the figures. It will be understood thatthe spatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as being “below” or “beneath”other elements or features would then be oriented “above” the otherelements or features. Thus, the exemplary term “below” can encompassboth an orientation of above and below. The device may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

FIG. 1 is a cross-sectional view of a bipolar transistor 100 having asplit collector region 150 in accordance with one or more embodiments.Bipolar transistor 100 includes a substrate 102. A deep n-well 104 is insubstrate 102. A field oxide 110 is over the deep n-well 104 and a gate140 is partly overlying field oxide 110. An emitter region 130 and splitcollector region 150 are on opposite sides of gate 140. Emitter region130 includes a pair of oppositely doped, i.e., one p-doped and onen-doped, regions 132 and 134 contained in a p-well 106. Split collectorregion 150 includes a highly doped central region 152 surrounded by alightly doped peripheral region 154 contained in an n-well 108.

In some embodiments, bipolar transistor 100 is a lateral insulated gatebipolar transistor (LIGBT). In some embodiments, bipolar transistor 100is a bipolar junction transistor (BJT) or another type of bipolartransistor.

In some embodiments, substrate 102 is a lightly doped substrate having afirst type of conductivity. Bipolar transistor 100 is an n-typetransistor, and thus, the substrate 102 includes a p-type siliconsubstrate (p-substrate) or an SOI (silicon on isolator) substrate. Insome embodiments, bipolar transistor 100 is a p-type transistor andsubstrate 102 includes an n-type dopant (n-substrate) or an SOI (siliconon isolator) substrate. In some embodiments, substrate 102 is asemiconductor wafer, such as a silicon wafer. Alternatively oradditionally, substrate 102 includes other semiconductors, such asgermanium, silicon carbide, gallium arsenic, indium arsenide, and indiumphosphide, in some embodiments. In some embodiments, substrate 102includes an alloy semiconductor, such as silicon germanium, silicongermanium carbide, gallium arsenic phosphide, and gallium indiumphosphide.

Deep n-well 104 is in substrate 102. In some embodiments, deep n-well104 is formed using an SOI wafer. In some embodiments, deep n-well 104is formed using an implantation process. In some embodiments, deepn-well 104 is formed in substrate 102 using a doped epitaxial process.In some embodiments, the dopant is added during the epitaxial process.In some embodiments, the dopant is implanted following the epitaxialprocess. In some embodiments, the n-type dopants include arsenic,phosphorous or other suitable n-type dopants.

P-well 106 is in substrate 102 over deep n-well 104. In someembodiments, p-well 106 is formed using an implantation process. In someembodiments, p-well is formed in substrate 102 using a doped epitaxialprocess. In some embodiments, the dopant is added during the epitaxialprocess. In some embodiments, the dopant is implanted following theepitaxial process. In some embodiments, the p-type dopants includeboron, boron difluoride, gallium or other suitable p-type dopants.

N-well 108 is in substrate 102 above deep n-well 104. In someembodiments, n-well 108 is formed using an implantation process. In someembodiments, n-well 108 is formed in substrate 102 using a dopedepitaxial process. In some embodiments, the dopant is added during theepitaxial process. In some embodiments, the dopant is implantedfollowing the epitaxial process. In some embodiments, the n-type dopantsinclude arsenic, phosphorous or other suitable n-type dopants. In someembodiments, a dopant species in n-well 108 is a same dopant species asin deep n-well 104. In some embodiments, the dopant species in n-well108 is different from the dopant species in deep n-well 104.

Field oxide 110 is over a top surface of substrate 102. In someembodiments, field oxide 110 includes a dielectric, such as siliconoxide, nitride, or other suitable insulating materials. In someembodiments, field oxide 110 is formed by a thermal oxide process.Substrate 102 is patterned to protect regions where field oxide 110 isundesirable and the substrate is subjected to a high temperature, forexample, about 800 degrees Celsius, in the presence of oxygen.

Emitter region 130 is formed in a top surface of p-well 106, across gate140 from split collector region 150. Emitter region 130 has twooppositely doped regions 132 and 134, both formed in the top surface ofp-well 106. A first region 132 of emitter region 130 has a first type ofconductivity. In some embodiments, the conductivity type of first region132 is the same as that of substrate 102. A second region 134 of emitterregion 130 has a second type of conductivity, which is the same as deepn-well 104. For example in FIG. 1, first region 132 includes p-typedopants such as boron, boron difluoride, gallium or other suitablep-type dopants. In some embodiments, a dopant species of first region132 is a same dopant species as substrate 102. In some embodiments, thedopant species of first region 132 is different from the dopant speciesof substrate 102. Second region 134 includes n-type dopants, such asarsenic, phosphorous, or other suitable n-type dopants. In someembodiments, a dopant species of second region 134 is a same dopantspecies as deep n-well 104. In some embodiments, the dopant species ofsecond region 134 is different from the dopant species of deep n-well104. In some embodiments, first region 132 includes n-type dopants andsecond region 134 includes p-type dopants. In some embodiments, emitterregion 130 is formed using ion implantation, diffusion, or anothersuitable formation method. A rapid thermal annealing (RTA) process isused to activate the implanted dopants, in some embodiments.

Gate 140 has a first portion overlying the p-well 106 and a secondportion overlying field oxide 110. In some embodiments, gate 140includes a gate dielectric and a gate electrode formed on the gatedielectric. The gate dielectric includes a silicon oxide layer suitablefor high voltage applications, in some embodiments. Alternatively, thegate dielectric includes a high-k dielectric material, siliconoxynitride, other suitable materials, or combinations thereof, in someembodiments. In some embodiments, the high-k material is selected frommetal oxides, metal nitrides, metal silicates, transition metal-oxides,transition metal-nitrides, transition metal-silicates, oxynitrides ofmetals, metal aluminates, zirconium silicate, zirconium aluminate,hafnium oxide, or combinations thereof. In some embodiments, the gatedielectric has a multilayer structure, such as one layer of siliconoxide and another layer of high-k material. In some embodiments, thegate dielectric is formed using chemical vapor deposition (CVD),physical vapor deposition (PVD), atomic layer deposition (ALD), thermaloxide, other suitable processes, or combinations thereof.

The gate electrode is disposed overlying the gate dielectric. In someembodiments, the gate electrode includes a doped or non-dopedpolycrystalline silicon (or polysilicon). Alternatively, the gateelectrode layer include a metal, such as Al, Cu, W, Ti, Ta, TiN, TaN,NiSi, CoSi, other suitable conductive materials, or combinationsthereof, in some embodiments. In some embodiments, the gate electrode isformed by CVD, PVD, ALD, plating, or other processes. In someembodiments, the gate electrode has a multilayer structure and is formedin a multiple-step process.

Split collector region 150 is formed in a top surface of n-well 108,across gate 140 from emitter region 130. Split collector region 150 hashighly doped central region 152 surrounded by lightly doped peripheralregion 154. Highly doped central region 152 has a same dopant type aslightly doped peripheral region 154. For example in FIG. 1, highly dopedcentral region 152 and lightly doped peripheral region 154 includep-type dopants such as boron, boron difluoride, gallium or othersuitable p-type dopants. In some embodiments, a dopant species of highlydoped central region 152 is a same dopant species as lightly dopedperipheral region 154. In some embodiments, the dopant species of highlydoped central region 152 is different from the dopant species of lightlydoped peripheral region 154.

A dopant concentration in highly doped central region 152 is about 50times to about 200 times greater than a dopant concentration in lightlydoped peripheral region 154. In some embodiments, the dopantconcentration in highly doped central region 152 is about 100 timesgreater than the dopant concentration in lightly doped peripheral region154. In some embodiments, the dopant concentration of lightly dopedperipheral region 154 ranges from about 5×10¹² ions/cm³ to about 5×10¹³ions/cm³.

In some embodiments, emitter region 130 is formed using ionimplantation, diffusion, or another suitable formation method. A rapidthermal annealing (RTA) process is used to activate the implanteddopants, in some embodiments.

FIG. 2 is a cross-sectional view of split collector region 150 for abipolar transistor in accordance with one or more embodiments. Splitcollector region 150 includes highly doped central portion 152surrounded by lightly doped peripheral portion 154. Split collectorregion 150 has a length L. In some embodiments, length L ranges fromabout 10 microns (μm) to about 30 μm. Highly doped central region 152has a length X parallel to length L. In some embodiments, a ratiobetween length X and length L ranges from about 0.8 to about 0.9.Lightly doped peripheral region 154 has a same width on both sides ofhighly doped central region 152. A length of lightly doped peripheralregion 154 on each side of highly doped central region 152 is equal to(L−X)/2.

FIG. 3A is a top view of a bipolar transistor 100 having a splitcollector region 150 in accordance with one or more embodiments. Bipolartransistor 100 includes emitter region 130 on a first side of gate 140and split collector region 150 on a second side of the gate opposite theemitter region. Bipolar transistor 100 also includes contacts 310electrically connected to emitter region 130, gate 140 and splitcollector region 150 to connect the bipolar transistor to other devicesin an integrated circuit.

FIG. 3B is a top view of split collector region 150 for a bipolartransistor in accordance with one or more embodiments. Split collectorregion 150 includes highly doped central portion 152 surrounded bylightly doped peripheral portion 154. Lightly doped peripheral portion154 completely surrounds highly doped central portion 152, and thehighly doped central portion is centered in the lightly doped peripheralportion.

Split collector region 150 provides an advantage of a collector regionhaving a uniform dopant concentration throughout. In comparison with auniform dopant concentration collector region, split collector region150 has a decreased turn-off time. A decrease in turn-off time meansthat when a signal applied to gate 140 indicates bipolar transistor 100should transition from a conductive state to a non-conductive state, thetransition occurs faster in structures which include split collectorregion 150. Table I below indicates that a turn-off time for a bipolartransistor having a split collector region is approximately as must as52% faster than a bipolar transistor having a uniform dopantconcentration collector region. The decreased turn-off time is providedfor different combinations of direct current voltage and current levels.By decreasing the turn-off time, power consumption of the bipolartransistor is decreased and artificial signal delays to other deviceswithin a current connected to bipolar transistor 100 are able to bereduced, thereby increasing an overall speed of the circuit.

TABLE I Turn-off Time Current 155 V 311 V 373 V Uniform dopant 0.5 A778.667 ns 570.667 ns 554.667 ns concentration   1 A 698.667 ns 448.000ns 432.000 ns collector region Split collector 0.5 A  373.0 ns  333.0 ns 344.0 ns region   1 A  361.0 ns  273.0 ns  271.0 ns Percent 0.5 A 52.1%41.6% 38.7% Reduction   1 A 48.3% 39.1% 37.3%

Decreasing an overall dopant concentration of a collector region wouldresult in a decreased conductivity of the bipolar transistor incomparison with a bipolar transistor having a higher dopantconcentration in the collector region. The decreased conductivityresults in an increased resistance to charge transfer from the emitterregion to the collector region. Increased resistance in turn increasespower consumption by the bipolar transistor. Decreasing an overall sizeof collector region will also reduce turn-off time, but the bipolartransistor will suffer similar negative results as decreasing the dopantconcentration. However, bipolar transistor 100 including split collectorregion 150 experiences a minor increase in resistance to chargetransfer. In comparison with a bipolar transistor having a collectorregion having a uniform high dopant concentration, bipolar transistorexhibits merely a 15% increase in resistance. The significant decreasein turn-off time combined with the small increase in resistance,increases applicability of bipolar transistor 100 to a wide variety ofcircuits.

FIG. 4 is flow chart of a method 400 of making a split collector regionin accordance with one or more embodiments. Method 400 begins withoperation 402 in which a first photoresist is patterned over a collectorregion of a bipolar transistor. The first photoresist is formed by asuitable process, such as spin-on coating, and then patterned to form apatterned photoresist feature by a proper lithography patterning method.In some embodiments, the pattern of the photoresist is developed by adry etching process, a wet etching process or another suitabledeveloping process. In some embodiments, a hard mask layer is formedbetween underneath the photoresist. The patterned photoresist layer isformed on the hard mask layer. The pattern of the photoresist layer istransferred to the hard mask layer. In some embodiments, the hard masklayer includes silicon nitride, silicon oxynitride, silicon carbide,and/or other suitable dielectric materials, and is formed using a methodsuch as CVD or PVD.

In some embodiments, the first photoresist is patterned to define alightly doped peripheral region, e.g., lightly doped peripheral region154 (FIG. 1), of the collector region. In some embodiments, the lightlydoped peripheral region has a length ranging from about 0.5 μm to about3 μm. In some embodiments, the first photoresist is patterned to definea highly doped central region, e.g., highly doped central region 152, ofthe collector region. In some embodiments, the highly doped peripheralregion has a length ranging from about 8 μm to about 27 μm. In someembodiments, the first photoresist is patterned to define an entirety ofthe collector region, e.g., split collector region 150. In someembodiments, the collector region has a length ranging from about 10 μmto about 30 μm.

Method 400 continues with operation 404 in which a first implant processis performed. In some embodiments, the first implant process includes anion implantation process followed by an anneal process to activeimplanted dopants. In some embodiments, the implantation processimplants p-type dopants, such as boron, boron difluoride, gallium orother suitable p-type dopants, into the bipolar transistor. In someembodiments, the implantation process implants n-type dopants, such asarsenic, phosphorous or other suitable n-type dopants, into the bipolartransistor.

In some embodiments where the first photoresist is patterned to definethe lightly doped peripheral region or the entirety of the collectorregion, the implantation process implants a dopant concentration rangingfrom about 5×10¹² ions/cm³ to about 5×10¹³ ions/cm³. In some embodimentswhere the first photoresist is patterned to define the highly dopedcentral region, the implantation process implants a dopant concentrationranging from about 5×10¹⁴ ions/cm³ to about 5×10¹⁵ ions/cm³.

In some embodiments, the first photoresist is removed following thefirst implant process. In some embodiments, the first photoresist isremoved using an ashing process, an etching process or another suitableremoval process.

FIG. 5A is a cross-sectional view of a split collector region 550following operation 404 in accordance with one or more embodiments.Split collector region 550 is similar to split collector region 150,similar elements have a same reference number increased by 400. In theembodiment of FIG. 5A, a first photoresist 560 is patterned to define alightly doped peripheral region 554. In some embodiments, firstphotoresist 560 is patterned to define an entirety of split collectorregion 550. In some embodiments, first photoresist 560 is patterned todefine a highly doped central region 552 (FIG. 5B).

Method 400 continues with operation 406 in which a second photoresist ispatterned over the collector region. The second photoresist is formed bya suitable process, such as spin-on coating, and then patterned to forma patterned photoresist feature by a proper lithography patterningmethod. In some embodiments, the second photoresist is formed using asame process as the first photoresist. In some embodiments, the secondphotoresist is formed using a different process from the firstphotoresist. In some embodiments, the pattern of the second photoresistis developed by a dry etching process, a wet etching process or anothersuitable developing process. In some embodiments, the second photoresistis developed using a same process as the first photoresist. In someembodiments, the second photoresist is developed using a differentprocess from the first photoresist. In some embodiments, a hard masklayer is formed between underneath the second photoresist. The patternedsecond photoresist is formed on the hard mask layer. The pattern of thesecond photoresist layer is transferred to the hard mask layer. In someembodiments, the hard mask layer includes silicon nitride, siliconoxynitride, silicon carbide, and/or other suitable dielectric materials,and is formed using a method such as CVD or PVD.

In some embodiments, the second photoresist is patterned to define alightly doped peripheral region, e.g., lightly doped peripheral region154 (FIG. 1), of the collector region. In some embodiments, the lightlydoped peripheral region has a length ranging from about 0.5 μm to about3 μm. In some embodiments, the second photoresist is patterned to definea highly doped central region, e.g., highly doped central region 152, ofthe collector region. In some embodiments, the highly doped peripheralregion has a length ranging from about 8 μm to about 27 μm.

In some embodiments where the first photoresist defines the lightlydoped peripheral region or the entirety of the collector region, thesecond photoresist defines the highly doped central region. In someembodiments where the first photoresist defines the highly doped centralregion, the second photoresist defines the lightly doped peripheralregion.

Method 400 continues with operation 408 in which a second implantprocess is performed. In some embodiments, the second implant processincludes an ion implantation process followed by an anneal process toactive implanted dopants. In some embodiments, the second implantationprocess implants p-type dopants, such as boron, boron difluoride,gallium or other suitable p-type dopants, into the bipolar transistor.In some embodiments, the second implantation process implants n-typedopants, such as arsenic, phosphorous or other suitable n-type dopants,into the bipolar transistor.

In some embodiments where the second photoresist is patterned to definethe lightly doped peripheral region, the implantation process implants adopant concentration ranging from about 5×10¹² ions/cm³ to about 5×10¹³ions/cm³. In some embodiments where the second photoresist is patternedto define the highly doped central region, the implantation processimplants a dopant concentration ranging from about 5×10¹⁴ ions/cm³ toabout 5×10¹⁵ ions/cm³.

In some embodiments, the second photoresist is removed following thesecond implant process. In some embodiments, the second photoresist isremoved using an ashing process, an etching process or another suitableremoval process.

FIG. 5B is a cross-sectional view of split collector region 550following operation 408 in accordance with one or more embodiments. Inthe embodiment of FIG. 5B, a second photoresist 570 is patterned todefine highly doped central region 552. In some embodiments, secondphotoresist 570 is patterned to define a lightly doped peripheral region554 (FIG. 5A).

One of ordinary skill in the art would recognize that additionaloperations before or after the described operations of method 400 areused to form a functional bipolar transistor. One of ordinary skill inthe art would also recognize that back end processes are also possibleto provide connection between the bipolar transistor and other deviceswithin a circuit.

The back end process include forming interconnect structures over thebipolar transistor. In some embodiments, the interconnect structureconnects to an emitter region and the split collector region. In someembodiments, the interconnect structure connects to a gate structure.

In some embodiments, the interconnect structure includes an interlayerdielectric (ILD) and a multilayer interconnect (MLI) structure in aconfiguration such that the ILD separates and isolates each metal layerfrom other metal layers. In furtherance of the example, the MLIstructure includes contacts, vias and metal lines formed on thesubstrate. In one example, the MLI structure may include conductivematerials, such as aluminum, aluminum/silicon/copper alloy, titanium,titanium nitride, tungsten, polysilicon, metal silicide, or combinationsthereof, being referred to as aluminum interconnects. Aluminuminterconnects may be formed by a process including physical vapordeposition (or sputtering), chemical vapor deposition (CVD), orcombinations thereof. Other manufacturing techniques to form thealuminum interconnect may include photolithography processing andetching to pattern the conductive materials for vertical connection (viaand contact) and horizontal connection (conductive line). Alternatively,a copper multilayer interconnect is used to form the metal patterns. Thecopper interconnect structure may include copper, copper alloy,titanium, titanium nitride, tantalum, tantalum nitride, tungsten,polysilicon, metal silicide, or combinations thereof. The copperinterconnect may be formed by a technique including CVD, sputtering,plating, or other suitable processes.

The ILD material includes silicon oxide. Alternatively or additionally,the ILD includes a material having a low dielectric constant, such as adielectric constant less than about 3.5. In one embodiment, thedielectric layer includes silicon dioxide, silicon nitride, siliconoxynitride, polyimide, spin-on glass (SOG), fluoride-doped silicateglass (FSG), carbon doped silicon oxide, Black Diamond® (AppliedMaterials of Santa Clara, Calif.), Xerogel, Aerogel, amorphousfluorinated carbon, Parylene, BCB (bis-benzocyclobutenes), SiLK (DowChemical, Midland, Mich.), polyimide, and/or other suitable materials.The dielectric layer may be formed by a technique including spin-on,CVD, or other suitable processes.

An aspect of this description relates to a method of making a bipolartransistor. The method includes patterning a first photoresist over acollector region of the bipolar transistor, the first photoresistdefining a first opening. The method further includes performing a firstimplantation process through the first opening. The method furtherincludes patterning a second photoresist over the collector region, thesecond photoresist defining a second opening different from the firstopening. The method further includes performing a second implantationprocess through the second opening, wherein a dopant concentrationresulting from the second implantation process is different from adopant concentration resulting from the first implantation process.

Another aspect of this description relates to a method of making abipolar transistor. The method includes forming a first well in acollector region of the bipolar transistor, wherein the first well has afirst dopant type. The method further includes forming a second well inan emitter region of the bipolar transistor, wherein the second well hasa second dopant type opposite the first dopant type. The method furtherincludes implanting a first dopant in the first well, wherein the firstdopant has the second dopant type. The method further includesimplanting a second dopant in the first well, wherein the second dopanthas the second dopant type, a dopant concentration of the second dopantin the first well is different from a dopant concentration of the firstdopant in the first well.

Still another aspect of this description relates to a method of making abipolar transistor. The method includes patterning a first photoresistover a collector region of the bipolar transistor, the first photoresistdefining a plurality of first openings exposing a first area of thecollector region. The method further includes performing a firstimplantation process through each of the plurality of first openings.The method further includes removing the patterned first photoresist.The method further includes patterning a second photoresist over thecollector region, the second photoresist defining a second openingexposing a second area of the collector region different from the firstarea. The method further includes performing a second implantationprocess through the second opening.

It will be readily seen by one of ordinary skill in the art that thedisclosed embodiments fulfill one or more of the advantages set forthabove. After reading the foregoing specification, one of ordinary skillwill be able to affect various changes, substitutions of equivalents andvarious other embodiments as broadly disclosed herein. It is thereforeintended that the protection granted hereon be limited only by thedefinition contained in the appended claims and equivalents thereof.

What is claimed is:
 1. A method of making a bipolar transistor, themethod comprising: patterning a first photoresist over a collectorregion of the bipolar transistor, the first photoresist defining a firstopening; performing a first implantation process through the firstopening; patterning a second photoresist over the collector region, thesecond photoresist defining a second opening different from the firstopening; and performing a second implantation process through the secondopening, wherein a dopant concentration resulting from the secondimplantation process is different from a dopant concentration resultingfrom the first implantation process.
 2. The method of claim 1, whereinpatterning the first photoresist comprises defining the first opening inan area of the collector region surrounding the second opening.
 3. Themethod of claim 1, wherein performing the second implantation processcomprises implanting a higher dopant concentration than the firstimplantation process, and the second implantation process comprisesimplanting a dopant concentration ranging from about 5×10¹² ions/cm³ toabout 5×10¹³ ions/cm³.
 4. The method of claim 1, wherein patterning thefirst photoresist comprises defining a plurality of first openings. 5.The method of claim 1, wherein performing the second implantationprocess occurs prior performing the first implantation process.
 6. Amethod of making a bipolar transistor, the method comprising: forming afirst well in a collector region of the bipolar transistor, wherein thefirst well has a first dopant type; forming a second well in an emitterregion of the bipolar transistor, wherein the second well has a seconddopant type opposite the first dopant type; implanting a first dopant inthe first well, wherein the first dopant has the second dopant type;implanting a second dopant in the first well, wherein the second dopanthas the second dopant type, a dopant concentration of the second dopantin the first well is different from a dopant concentration of the firstdopant in the first well.
 7. The method of claim 6, wherein implantingthe second dopant comprises implanting a same species as a species ofthe first dopant.
 8. The method of claim 6, wherein implanting thesecond dopant comprises implanting a different species from a species ofthe first dopant.
 9. The method of claim 6, wherein implanting thesecond dopant occurs after implanting the first dopant, and the dopantconcentration of the second dopant in the first well ranges from about5×10¹² ions/cm³ to about 5×10¹³ ions/cm³.
 10. The method of claim 6,further comprising oxidizing an area between the collector region andthe emitter region to form a field oxide.
 11. The method of claim 10,further comprising forming a gate structure overlapping the field oxide.12. The method of claim 6, wherein implanting the first dopant comprisesimplanting the first dopant to have the dopant concentration of thefirst dopant about 50 times to about 200 times greater than the dopantconcentration of the second dopant.
 13. The method of claim 6, whereinimplanting the second dopant occurs prior to implanting the firstdopant, and implanting the first dopant comprises implanting the firstdopant into an area of the first well including the second dopant. 14.The method of claim 6, further comprising: performing a first annealfollowing implanting the first dopant; and performing a second annealfollowing implanting the second dopant.
 15. The method of claim 6,wherein forming the first well comprises performing an implantationprocess.
 16. The method of claim 6, wherein forming the first wellcomprises: recessing a substrate to define a recess; and epitaxiallygrowing a doped material in the recess.
 17. A method of making a bipolartransistor, the method comprising: patterning a first photoresist over acollector region of the bipolar transistor, the first photoresistdefining a plurality of first openings exposing a first area of thecollector region; performing a first implantation process through eachof the plurality of first openings; removing the patterned firstphotoresist; patterning a second photoresist over the collector region,the second photoresist defining a second opening exposing a second areaof the collector region different from the first area; and performing asecond implantation process through the second opening.
 18. The methodof claim 17, further comprising annealing the bipolar transistorfollowing performing the first implantation.
 19. The method of claim 17,wherein patterning the second photoresist comprises exposing the secondarea surrounded by the first area.
 20. The method of claim 17, whereinthe second implantation process comprises implanting a dopantconcentration ranging from about 5×10¹² ions/cm³ to about 5×10¹³ions/cm³